Tire Rub and Adjusting the Panhard Bar

[snettleship]

Hello, I was racing at VIR this weekend with a new set of Avon ACB 9 tires I got over the winter. Anyway, these are a little wider than the Hoosier Street TDs I previously used. I have a rub on the 'outside left rear' and 'inside left rear'. Basically on hard right hand corners. Here are the stat's on the tires:

 

SIZE--------APPROV RIMS--Measured RIM--TYRE DIA--SECTION WIDTH--THREAD WIDTH

ACB 9

6.5/23.0-13---5.5-6.5----------5.5----------22.7----------8.5---------------6.7

Hoosier

A70/13-------5.0-7.0----------5.0-----------22.4---------7.5---------------6.2

 

So they are rubbing on the shoulders. I'd like to be able to adjust the panhard bar on my Series 4 Seven. But, I need the suspension tuning for dummies course. They also use rubber bushings and was wondering with going to polly ones would reduce the sway. Anyway, and detailed information would be appreciated. I'm enclosing some pics of the setup. Thanks as always.

 

Scott

[Ian7]

Not sure that bushings are your primary problem. If the axle is moving laterally under cornering loads, then so is the hub and wheel center by an equal amount, no? Is the tire rubbing on a piece that moves laterally with the axle, or a fixed point on the sprung part of the chassis that doesn't move laterally under load?

I think what you are seeing is a wider and possibly more flexible sidewall that is now hitting where the previous one didn't. If the Avon's have more grip than the hoosiers, then this will be exasperated by higher cornering loads, making deflections greater, etc etc.

You could go to a lot of trouble and expense to replace the bushings to cure a 1/8" deflection and still find it rubs.

How about rigging up some kind of a tell-tale pointer to measure axle deflection first before going to far with this?

[MHKflyer52]

Hi Scott,

 

Look at the top or your shock as I have seen the spring under compression such as in a turn grow and then rub the sidewall of the tire due to it being a contained coil under load....the opposite happens when the spring is stretched it gets smaller in diameter most of the time. If the spring has rub marks on it then you might want to look at shimming the shock away from the tire to help relive the rub or get stiffer springs or smaller tires.

 

I agree with Ian7 about checking for other causes. One way is to rub some white chalk on the sidewall or were ever it is rubbing and due some right and left turns in a parking lot and get out and see if you can see what is rubbing or moving as the chalk will help find what is doing it by leaving some residue on the culprit.

 

Just a suggestion and in no way to be taken as I knew or know what I am talking about.

[snettleship]

The rub is on the fiberglass bodywork on both sides. With the S4, the bodywork tub us a single formed piece that is attached to the frame. The suspension hangs from the top and the trailing arms are also connected to the frame. The panhard bar connects the with the rear trailing arm on the side of the driver. Then connects to the frame directly under the drive shaft.

 

I've enclosed an image file. So, I believe the lateral load on the drivers' side.

 

Scott

[slngsht]

If you are hitting on both sides, you can't make any lateral adjustments to help that.

 

also, I don't see a panhard rod in the pics.

 

One option you have is to stiffen up the suspension. Another one would be to use a small spacer on the wheels.

[rotus8guy]

OK, not to get in a semantics argument,

but that's not a panhard rod. The S4 Seven has (on one side or the other, book says right, your photos show left) a triangulated trailing arm. It's not adjustable (as far as I know) so if your tires rub, spacers, and or less wide tires would be your best bet.

 

hth,

 

john

[snettleship]

Yes, I agree, this is not technically a panhard bar. In the diagram, you can see a triangulated bar that extend from the chassis under the drive shaft to the driver's side trailing arm. In my case the driver's side is on the left, so I assume that's why mine extends toward the left rear wheel.

 

I think the most prudent thing to do is to try and look into wheel backspace and spacers. I have sort of a variety of Panasport and knock-off panasports (9 in all). A space would work on the right rear since the rub is on the inside body work.

 

Any advice for wheel spacers? Never used them before and want to make sure it will take the load of cornering. Should I shy away from the universal type? Thanks.

[Ian7]

Here's my $.02 on spacers.... only use them if you cant afford narrower tires or rims with different backspacing. Depending on your hub and wheel design, the spacers will possibly defeat any centering action between the hub and wheel center, which induces bending loads into the studs. Also, depending on the studs and spacer thickness you choose, you may have less than ideal number of threads engaged.

Dynamically, you are widening the rear track on a car that generally understeers to start with, and this will only get worse.

Undoubtably, others will jump in and say "I've used spacers for years...". :-)